1. Field of the Invention
The present invention relates to an optical head which optically writes or reads information to/from plural kinds of information recording media such as optical discs, a diffraction device and an objective lens used for the optical head, an optical disc apparatus including the optical head, and a computer, an optical disc player, and an optical disc recorder including the optical disc apparatus.
2. Description of the Related Art
In recent years, with a blue-violet semiconductor laser in practical use, a Blu-ray Disc (hereinafter, a BD) as a high density and large capacity optical information recording medium (hereinafter, referred to as an optical disc) having the same size as that of a CD (Compact Disc) and a DVD (Digital Versatile Disc), has been put to practical use. The BD is an optical disc which has a protective material having a thickness of about 0.1 mm and performs write or read using a blue-violet laser light source having a wavelength of about 400 nm and an objective lens having a numerical aperture (NA) up to 0.85.
An HD DVD which has a protective material having a thickness of 0.6 mm and a blue-violet laser light source having a wavelength of about 400 nm and an objective lens having a numerical aperture of 0.65 are used has also been put to practical use.
There has been proposed an optical head which has compatibility and focuses a laser beam onto information recording surfaces of such optical discs having different protective material thicknesses using one objective lens so as to write or read information.
Each of Japanese Patent Application Laid-Open (JP-A) No. H7-98431 and Japanese Patent Application Laid-Open (JP-A) No. H10-10308 disclose an optical head having a focusing optical system which can focus a laser beam onto optical discs having different protective material thicknesses to diffraction limit.
FIG. 33 shows a configuration example of a conventional optical head disclosed in the Japanese Patent Application Laid-Open (JP-A) No. H7-98431. In FIG. 33, an optical head 130 includes a light source 101 for emitting a red laser beam, a beam splitter 103, a collimate lens 104, a hologram lens 105, an objective lens 106, a detecting lens 108, and a light receiving device 109. A DVD 70 is an optical disc having a protective material thickness of 0.6 mm.
The operation of the optical head 130 which writes or reads information to/from the DVD 70 will be described. The red laser beam emitted from the light source 101 passes through the beam splitter 103 and is then converted to a substantially parallel beam by the collimate lens 104. The red laser beam passes through the hologram lens 105 and is then focused as a light spot onto an information recording surface of the DVD 70 beyond a protective material by the objective lens 106. The red laser beam in a returning path reflected on the information recording surface of the DVD 70 passes through the objective lens 106, the hologram lens 105, and the collimate lens 104 in the same optical path as an advancing path. The red laser beam in the returning path is reflected by the beam splitter 103 and is then given predetermined astigmatism by the detecting lens 108. Finally, the red laser beam in the returning path is led to the light receiving device 109 so as to generate an information signal and a servo signal.
The operation of the optical head 130 for performing write or read to/from a CD 80 as an optical disc having a protective material thickness of 1.2 mm will be described with reference to FIG. 34. The red laser beam emitted from the light source 101 passes through the beam splitter 103 and is then converted to a substantial parallel beam by the collimate lens 104. The red laser beam is diffracted by the hologram lens 105 and is then focused as a light spot onto an information recording surface of the CD 80 beyond a protective material by the objective lens 106. The red laser beam in a returning path reflected on the information recording surface of the CD 80 passes through the objective lens 106, the hologram lens 105, and the collimate lens 104 in the same optical path as an advancing path. The red laser beam in the returning path is reflected by the beam splitter 103 and is then given predetermined astigmatism by the detecting lens 108. Finally, the red laser beam in the returning path is led to the light receiving device 109 so as to generate an information signal and a servo signal.
A focus error signal for performing write or read to/from the DVD 70 and the CD 80 can be detected by using an astigmatism method, etc. of detecting a focal spot given the astigmatism by the detecting lens 108 in a four-part split light receiving pattern in the light receiving device 109. A tracking error signal can be detected by using what is called a three-beam method, a differential push-pull method (DPP method), and the like which use a main beam and a sub-beam generated by a diffraction grating (not shown).
The functions of the hologram lens 105 and the objective lens 106 will be described in detail with reference to FIGS. 35 and 36.
The hologram lens 105 has a grating pattern 105a as shown in FIG. 35 for focusing a very small light spot onto each of the DVD 70 and the CD 80. The diffraction efficiency of a plus-first-order diffraction light of the hologram lens 105 is less than 100%. The hologram lens 105 is designed in such a manner that a transmission light (hereinafter, in the present invention, a non-diffracted transmission light can also be represented as a zero-order diffraction light and the transmission light is handled as one of diffraction lights) has sufficient intensity. The hologram lens 105 is blazed so as to increase the sum of the amount of the zero-order diffraction light and a plus-first-order diffraction light. The light utilization efficiency can be high.
The objective lens 106 has an NA of 0.6 and is designed so as to form a focal spot onto the DVD 70 having a protective material with a thickness of 0.6 mm to diffraction limit, as shown in FIG. 36A, when the laser beam passed through the hologram lens 105 without being diffracted (that is, the zero-order diffraction light) is incident.
As shown in FIG. 36B, the plus-first-order diffraction light diffracted by the hologram lens 105 is focused onto the CD 80 by the objective lens 106. Here, the plus-first-order diffraction light is aberration-corrected so as to form a focal spot onto the CD 80 having a protective material thickness of 1.2 mm to diffraction limit.
The hologram lens 105 for diffracting part of the incident light is combined with the objective lens 106 so as to realize a bifocal lens which can form a focal spot focused onto each of the optical discs having different protective material thicknesses to diffraction limit.
Since the hologram lens 105 has lens action, the positions of two focal points in the optical axis direction are different. Thus, when a light spot formed on one of the focal points is used to write or read information, a light spot focused on the other focal point is enlarged, which does not affect write or read of information.
With such optical head 130, information can be written or read to/from different kinds of optical discs using one objective lens.